Replacement of automotive rubber hoses (such as those used for delivery of gasolines) by more durable and lighter polyamide resin hoses is under current review. However, polyamide resin hoses are not as flexible as rubber hoses, and efforts are being made to improve the flexibility of polyamide resins.
The conventional approach for improving the flexibility of polyamide resins is either by compounding a plasticizer with the polyamide resin or by copolymerizing or blending rubber with the polyamide resin. However, the first method has not a great practical feasibility because the plasticizer evaporates under heated conditions or it is extracted with oils or solvents (e.g. gasoline), and in either case, the flexibility of the polyamide resin is significantly reduced. When the polyamide resin is copolymerized with rubber, the melting point of the resin is usually reduced and this is not desired for the intended use of the polyamide resin under elevated temperatures. As a rubber material to be blended with the polyamide resin, acrylonitrile-butadiene copolymer rubbers (hereunder referred to as nitrile rubber) are particularly selected because the blend must withstand the environment of use where it is brought into contact with oils or solvents (e.g. gasoline). U.S. Pat. No. 4,173,556 proposed a method of heating the blend of nitrile rubber and polyamide resin in the presence of a crosslinking agent so that the nitrile rubber being crosslinked is dispersed in the polyamide resin. However, in either method, the inherently good heat-aging characteristics of the polyamide resin are greatly impaired when it is blended with the nitrile rubber.
Therefore, the present inventors have made various studies to produce a polyamide-rubber blend composition having not only good heat-aging characteristics but also high strength, great resistance to cracking and gasolines, and improved flexibility. This object can be achieved by the present invention which will be described hereinafter in detail.